RSS stands for "Really Simple Syndication" - it allows a site to publish new articles and updates in a machine-readable, standard format. Which all sounds pretty dull, but can actually be quite handy when trying to keep abreast of all the latest developments on any website that supports them.
On their own, RSS feeds are not terribly useful or interesting to most people. But remember the part where I said "machine-readable"? *That's* why they're useful. Computer programs have been developed which can download, process and report back on your selected feeds, without you going to check the site yourself. The upshot of this is that if any new articles are posted, or any new comments written, your computer can tell you. This massively reduces the amount of time you spend either looking at a site that's not been updated, or contains nothing new to interest you.
You can get RSS readers for all platforms. Here are some examples:
Here are some links dedicated to RSS around the web: